After finishing my wafer clean at wbdiff, I found that I couldn't find an empty DIFF box to return the cassette to. I looked in every box and found that the enclosed cassette in each box had no COLORED button (i.e. seemingly OK). However, upon closer inspection I noticed that one of the cassettes had no button at all. At the same time, I noticed that there's an extra empty ETCH box by wbnonmetal. I wonder if the yellow button fell off an ETCH cassette and someone mistook it for a DIFF cassette and put it in the wrong box.